#124 NFB Resistor

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Structo
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Location: Oregon

Re: #124 NFB Resistor

Post by Structo »

vibratoking wrote:There is an effective parallel capacitance and effective series inductance associated with resistors. For CC, CF, MF types used in most guitar amps, the effective reactances are neglible IMO. This is a can of worms open for a lot of debate such as "you can't measure it, but I can hear it". Here is a link to some data that shows the effect of these reactances occurs in the MHz and above regions. I doubt that this would be important with regard to the NFB loop in a guitar amp.

http://ieeeb.okstate.edu/lecturenotes/E ... ematic.pdf
I wonder how much a audio frequency is affected by those factors.

It also depends on the resistor construction.
I have read that carbon comp resistors have much less capacitance and inductor properties than film type resistors which are often times spiral cut onto a ceramic substrate.

In each of those graphs the line was flat until approx 1 MHz.
I think it's safe to say that most guitar amps, even taking in account of harmonics won't (hopefully) be dealing with those frequencies.

I'm sure the guys that build and listen to Hi Fi amps take those properties into consideration, perhaps obsessively so. :D

It's funny that we (guitar guys) purposely make an amplifier distort the guitar signal, overdriving tube stages.

But, it's the type, sound and feel of this distortion that we obsess about, trying to make it pleasing to the ear.
So on one hand we want a nice clean, non-distorted sound ala blackface Fender then with a push of a footswitch we want that creamy, horn like overdrive that a Dumble is famous for.

Fun stuff! :D
Tom

Don't let that smoke out!
mobisimo
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Re: #124 NFB Resistor

Post by mobisimo »

I've read a recommendation that in the (sensitive) grid stopper position a resistor with little inductance is important.
I employed the "hifi" trick of testing resistors with a magnet and while the metal films and carbon comps didn't stick, the carbon films did.
I'll be ordering both carbon films and carbon comps (either ohmite little demons and/or kamata from mouser http://www.mouser.com/catalog/644/usd/722.pdf) to see if this seems to make a difference and will report back. Maybe try some metal films also just to see. I think I saw a pic recently of a 70s hybrid (?) that used cc grid resistors..
Has anyone tried this already?
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renshen1957
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Re: #124 NFB Resistor

Post by renshen1957 »

mobisimo wrote:I've read a recommendation that in the (sensitive) grid stopper position a resistor with little inductance is important.
I employed the "hifi" trick of testing resistors with a magnet and while the metal films and carbon comps didn't stick, the carbon films did.
I'll be ordering both carbon films and carbon comps (either ohmite little demons and/or kamata from mouser http://www.mouser.com/catalog/644/usd/722.pdf) to see if this seems to make a difference and will report back. Maybe try some metal films also just to see. I think I saw a pic recently of a 70s hybrid (?) that used cc grid resistors..
Has anyone tried this already?
Hi,

You might consider RCD CC resistors (made in the USA) which have the specifications for distortion of the old AB and Stackpole. I have no difficulty of locating NOS AB from my sources.

Best Regards,

Steve
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